A variety of approaches have been used for the purpose of detecting the radioactivity levels in large numbers of radioactive samples. The sample material to be analyzed typically consists of some biological substance containing a source of radioactivity. The extent of the existence of the radioactivity conveys certain information useful, for example, in medical research or diagosis. In liquid scintillation analysis, for example, laboratory animals ingest certain substances labeled with radioactive tracers, typically carbon-14 or tritium. The proclivity of these substances to migrate to certain biological organs or parts thereof is indicative of the degree to which the particular substance under study seeks the organ involved, or is indicative of the medical status of the organ. In any event, portions of the organ may thereafter be dissolved in a solvent to which a liquid scintillator is added. Beta rays from the radioisotopes in the organ cause flashes of light to be emitted by the scintillator in a transparent vial or test tube. The flashes of light, or scintillations, are counted using one or more photomultiplier tubes in order to determine the level of radioctivity of the sample under study. Typical laboratory research results in a requirement for processing several hundred liquid scintillation samples at a time.
Similarly, gamma emitting samples are required in large numbers in clinical medical diagnosis. One purpose for which such samples are typically prepared is for use in radioimmunoassay. In radioimmunoassay, or other competitive binding types of assays, an antigen is added to a serum containing a specific antibody. The antigen and antibody combine and precipitate from the solution. Some of the antigen used in the reaction is combined with or "labeled" with a radio-isotype, typically iodine-125. The radioisotope is chosen in such a manner that it will not interfere with the tendency for the antigen to combine with specific antibody. The antigenantibody complex may be separated from the excess antigen, and the portion of this complex which is radioactive will determine the amount of antigen initially present. A description of a specific application of radioimmunoassay may be found in an article by Catt, Niall, and Tregear, "Solid-Phase Radioimmunoassay of Human Growth Hormone", Biochem. Journal, 100, pp. 316-336 (1966).
While measurement of radioactive samples has been described heretofore in terms of its use in biological research and clinical laboratories, such applications are not limiting as to the scope of the present invention. Radioactive samples may be measured according to the present invention in connection with other types of radioactivity determinations. For example, the present invention could be applied to samples derived from gas or liquid chromatography units, or for the purpose of determining mineral content in geological samples, for determining wear in moving parts in the testing of engines, for charting air and sea currents in weather prediction, and for a host of other applications. The common element in each of these applications is that a number of separate and discrete radioactive samples are produced for analysis.
It is an object of the present invention to concurrently measure the radioactivity level in a plurality of radioactive samples. Heretofore, the measurement of radioactive samples has taken place sequentially is typified by the planchett counter disclosed in U.S. Pat. No. 2,843,753, and by the serpentine conveyor system disclosed in U.S. Pat. No. 3,206,006 for use in liquid scintillation and gamma counting. In this type of prior art radioactivity measurement, discrete samples are transported to a sample measurement station where the level of radioactivity of each sample is measured. When measurement is complete for one sample, that sample is advanced and replaced by a subsequent sample. Some attempts have been made to concurrently analyze the radioactivity from more than one sample. For example, U.S. Pat. No. 3,723,736 discloses a means and method for analyzing a plurality of liquid scintillation samples concurrently. However, this system still contemplates a serpentine conveyor to bring groups of samples to a sample measurement station where each of the samples in a group are concurrently measured. The disclosure in U.S. Pat. No. 3,855,473 illustrates a system in which the sample detection station is sequentially brought to a group of samples to concurrently measure each of the samples in that group. Again, as in the other systems described, the radioactivity detecting mechanism must advance sequentially measuring only a very small group of sample concurrently, and an elaborate mechanical apparatus for effecting relative and sequential movement between samples and a sample measurement station is necessary.
Accordingly, it is an object of the present invention to measure concurrently a plurality of discrete radioactive samples in a manner which eliminates the sequential processing of single samples or groups of samples within a total set. The elimination of sequential and recurring cycles of sample measurement provides enormous savings in time in sample analysis. That is, if an average of n seconds is required to analyze a single sample, and three hundred samples must be analyzed, the device of the present invention will analyze all of these samples in only slightly more than n seconds. Conversely, a sample measurement device that measures samples in groups of three would require 100n seconds for the same processing, while a sample analyzing device measuring single samples sequentially would require 300n seconds.
It is a further object to concurrently measure the radioactivity of a plurality of samples with virtually no increase in measuring time required for additional samples. That is, continuing the previous examples, the device of the present invention could measure 500 samples in approximately n seconds, practically the same amount of time as would be required to measure 300 samples. Conventional devices would require an increased processing time proportional to the number of additional samples.
Another object of the present invention is the elimination of the requirement for moving either the samples or the sample measurement station during the sample measuring process. The elimination of sample movement does away with problems of sample spillage and breakage, and increases reliability by eliminating entirely the complex and expensive mechanical transport system characteristic of conventional devices. Accordingly, the present invention does not require a serpentine conveyor system nor a mechanism for moving the sample measuring station relative to the samples to be measured. Instead, in some forms of the invention, the samples are fixed relative to the radiation detecting mechanism for the duration of the processing cycle. The samples need only be positioned for measurement and removed once counting is complete for all of the samples. There is thus no delay incurred in sample measurement for manipulation of either the samples or the sample detecting station between cycles of measurement.
A further object of the preferred practice of the invention is to increase the accuracy of the results obtained. Accordingly, collimation is provided so that electrical pulses caused by radiation from one sample are not attributed to a different sample. A collimator interposed between the samples and the detector will aid in the prevention of erroneous attribution of signal pulses. In addition for the important and specific application of the present invention where the radioisotope to be measured is iodine-125, and where a plurality of scintillation crystals are utilized as the scintillation means, there is disclosed herein a unique technique of correcting for certain inherent problems. Specifically, it is well known that scintillation crystals formed of the same material and of identical geometric size and configuration still will have variances in their optical transmission properties. It is also known that such physically identical crystals exhibit slight differences in responsiveness to incident radiation. Moreover, individual samples to be measured will exhibit certain differences which will affect the detected count rate of radioactive events. These effects result from such differences as variations in sample volume and variation in wall thickness and radiation absorption in the sample containers.
Accordingly, it is a further object of the presentation to provide a method and means for correcting for the foregoing problems so that a corrected count of actual radioactive disintegrations can be determined for each sample despite the existence of such differences and variations.